m68knommu: fix 'screen_bits' defined but not used

The 'screen_bits' LCD initializer bit array is only required if
CONFIG_INIT_LCD is enabled, otherwise we get the following warning:

    arch/m68k/68000/screen.h:4:22: warning: 'screen_bits' defined but not used [-Wunused-variable]
     static unsigned char screen_bits[] = {
                          ^~~~~~~~~~~

Make the inclusion of 'screen_bits' conditional on CONFIG_INIT_LCD.
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reported-by: kernel test robot <lkp@intel.com>

arch/m68k/68000/screen.h

 /* Created with The GIMP */
+#ifdef CONFIG_INIT_LCD
 #define screen_width 320
 #define screen_height 240
 static unsigned char screen_bits[] = {
@@ -802,3 +803,4 @@ static unsigned char screen_bits[] = {
    0x93, 0x10, 0xe2, 0x11, 0x00, 0x94, 0x22, 0x52, 0x69, 0x53, 0x52, 0x45,
    0x49, 0x22, 0xa4, 0x4a, 0x55, 0x29, 0x2a, 0xa4, 0x52, 0x42, 0xaa, 0xa5,
    0x52, 0xa8, 0xaa, 0x55, 0x4a, 0xab, 0xa9, 0x4a, 0x54, 0x49, 0x32, 0x24 };
+#endif /* CONFIG_INIT_LCD */